Article pubs.acs.org/JPCC
CO Oxidation on PdO(101) during Temperature-Programmed Reaction Spectroscopy: Role of Oxygen Vacancies Feng Zhang,† Li Pan,‡ Tao Li,† John T. Diulus,† Aravind Asthagiri,‡ and Jason F. Weaver*,† †
Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, United States William G. Lowrie Chemical & Biomolecular Engineering, The Ohio State University, Columbus, Ohio 43210, United States
‡
S Supporting Information *
ABSTRACT: We investigated the oxidation of CO on PdO(101) using temperature-programmed reaction spectroscopy (TPRS), reflection absorption infrared spectroscopy (RAIRS), and density functional theory (DFT). We find that about 71% of the CO molecules adsorbed in a saturated layer on PdO(101) transform to CO2 during TPRS, with the CO2 desorbing in two main features centered at 330 and 520 K. RAIRS shows that CO molecules initially adsorb in an atop configuration on coordinatively unsaturated (cus) Pd sites of PdO(101) located next to Ocus atoms, yielding a RAIRS peak at 2135 cm−1, and that the oxidation of these species produces the CO2 TPRS peak at 330 K. Concurrent with reaction, a large fraction of CO molecules migrates to atop-Pdcus sites located next to Ocus atom vacancies (Ov) that are created during reaction, as evidenced by the appearance of a RAIRS peak centered at ∼2085 cm−1. Our RAIRS measurements demonstrate that oxidation of the CO-Pdcus/Ov species is responsible for the CO2 TPRS peak at 520 K, and further show that oxygen atoms from the subsurface readily fill Ov sites as CO molecules vacate the Pdcus/Ov sites above about 400 K. DFT calculations show that a strong enhancement in binding (∼70 kJ/mol) is responsible for the rapid migration of CO molecules from Pdcus/ Ocus sites to Pdcus/Ov sites as the PdO(101) surface is reduced at low temperature. DFT also predicts that both CO species can access facile pathways for oxidation on PdO(101) via reaction with Ocus atoms, wherein the apparent reaction barriers are nearly identical in each pathway.
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INTRODUCTION Oxides of the late transition-metals play an important role in many applications of oxidation catalysis. Under oxygen-rich conditions, metal oxide layers can develop on catalytic metal surfaces. The formation of metal oxide layers usually alters the surface reactivity significantly due to differences in the chemical properties of the metal oxide compared with the metal surface. The oxidation of CO over late transition-metals has received considerable attention,1−12 due, in large part, to interest in understanding observations that the CO oxidation rate increases when metal oxide layers begin to form on late transition metals. For example, pioneering work by Over et al. demonstrates that a RuO2(110) thin-film develops on Ru(0001) under sufficiently oxidizing conditions, and that the RuO2(110) surface is highly active toward CO oxidation.13 Subsequent investigations have provided detailed information about the mechanism(s) for CO oxidation on RuO2(110).14−16 The microscopic understanding that has emerged from these prior studies may be broadly applicable to understanding the surface chemistry of late transition-metal oxide surfaces; however, investigations of CO oxidation on other oxide surfaces are needed to determine common aspects of the CO oxidation mechanism among different oxides. Prior studies14−16 demonstrate that coordinatively unsaturated (cus) Ru and O atoms are responsible for the high activity © 2014 American Chemical Society
of the RuO 2 (110) surface toward CO oxidation. On RuO2(110), Rucus atoms form rows that are separated by rows of bridging O atoms (Obr). Both of these species expose a single coordination vacancy relative to the bonding of Ru and O atoms within bulk RuO2. Adsorbed CO binds strongly on top of the Rucus atoms (Ead ∼ 110 kJ/mol), and reacts readily with neighboring Obr atoms.17−19 For example, experiments reveal that reaction between COcus and Obr species on RuO2(110) produces a CO2 peak centered at about 330 K during temperature-programmed reaction spectroscopy (TPRS), and that all of the Obr atoms can be removed by continuously CO dosing at 300 K in ultrahigh vacuum (UHV).19 Prior studies also show that CO molecules bind more strongly on Obr vacancies than on Rucus sites (Ead ∼ 170 versus 110 kJ/mol) of RuO2(110).20,21 The creation of strongly binding bridge sites introduces new elementary steps into the overall CO oxidation process that are not available on the pristine RuO2(110) surface. Recent investigations using reflection absorption infrared spectroscopy (RAIRS) and kinetic Monte Carlo (kMC) simulations have provided new insights for understanding CO Received: September 16, 2014 Revised: November 15, 2014 Published: November 18, 2014 28647
dx.doi.org/10.1021/jp509383v | J. Phys. Chem. C 2014, 118, 28647−28661
The Journal of Physical Chemistry C
Article
The FTIR system employed in this study consists of a MIR source (Bruker Tensor 27), a set of mirrors and lenses and an external liquid N2 cooled HgCdTe (MCT) detector. Outside of the UHV chamber, the MIR beam travels within a sealed box which is purged continuously with carbon dioxide and waterfree compressed air. In the purge box, a set of flat mirrors are used to direct the MIR beam onto a parabolic mirror that focuses the beam. The focused MIR beam enters the UHV chamber through a differentially pumped KBr window and then reflects from the sample surface at angle of ∼80° relative to the surface normal. The reflected MIR beam exits the UHV chamber through another KBr window and is directed onto the MCT detector within a second purge box. We provide more details of the RAIRS measurements performed in this study in the Results and Discussion section. We averaged 512 scans at a resolution of 4 cm−1 for all RAIR spectra reported here. The Pd(111) crystal used in the present study is a circular disk (8 mm × 1 mm) spot-welded to W wires and attached to a copper sample holder that is in thermal contact with a liquid nitrogen cooled reservoir. A type K thermocouple is spotwelded to the backside of the crystal to measure the sample temperature. The sample is resistively heated and the temperature is controlled using a PID controller that adjusts the output of a dc power supply. In this setup, we are able to maintain or linearly ramp the sample temperature from 85 to 1250 K. Initial sample cleaning consisted of cycles of sputtering with 600 eV Ar+ ions at a surface temperature of 900 K, followed by annealing at 1100 K for 5 min. Subsequent cleaning involved routinely exposing the sample to an atomic oxygen beam for 30 min at 856 K, followed by flashing the sample to 923 K to desorb oxygen and carbon oxides. As discussed previously,30 we limited the sample temperature to 923 K in order to maintain saturation of oxygen in the subsurface reservoir, which ensures reproducibility in preparing the PdO(101) thin films. We considered the Pd(111) sample to be clean when we could not detect contaminants with Auger electron spectroscopy (AES) and did not observe CO production during temperature-programmed desorption after oxygen adsorption. We generate a PdO (101) thin film by exposing the Pd(111) surface to an ∼23 ML dose of oxygen atoms at 500 K, where we define 1 ML as equal to the Pd(111) surface atom density of 1.53 × 1015 cm−2. This procedure generates a PdO(101) surface with identical surface structure and chemical properties to those studied previously.31−33 We provide details of the PdO(101) surface structure in the Computational section. During atomic oxygen exposure, the Pd(111) surface is positioned ∼20 mm from the final collimating aperture and is orientated 45° from the axis of the atomic oxygen beam. LEED observations suggest that this preparation procedure generates a PdO(101) film that uniformly covers the Pd(111) surface. After preparing the PdO(101) film, the sample is cooled to 95 K and then exposed to 9 L (Langmuir) of CO which is found to be sufficient to saturate PdO(101). After the CO exposure, we collected TPRS spectra by positioning the COsaturated PdO(101) surface in front of a shielded quadrupole mass spectrometer (Hiden) at a distance of ∼5 mm. The sample temperature increases to 650 at 1 K/s as the mass spectrometer monitors CO and CO2 desorption from the surface. The CO desorption yield measured in the TPRS spectrum is estimated by scaling CO TPD spectra obtained after saturating Pd(111) with CO at 200 K, assuming that the saturation coverage is 0.66 ML.34 The CO2 yield is estimated by
oxidation on RuO2(110). Farkas et al. have shown that the C− O stretching frequency of adsorbed CO is highly sensitive to the local binding environment on RuO2(110), and demonstrate that RAIRS can therefore be used to study the evolution of local adsorbate configurations during CO adsorption and reaction on RuO2(110).22 Their work reveals that two distinct types of CO-rich domains (denoted as COcus/CObr domains) coexist with O-rich domains under steady-state conditions, and that CO oxidation can occur by several pathways depending on the reaction conditions. Farkas et al. find that the reaction of CObr with Ocus species makes the dominant contribution to the CO2 production rate at stoichiometric feed conditions, thus demonstrating that Obr vacancies can play a central role in mediating CO oxidation on RuO2(110).23 Investigations of CO oxidation on Pd surfaces also show that increases in catalytic activity coincide with the onset of metal oxide formation. For example, researchers have found that the CO oxidation activity of Pd(100) significantly increases when a well-ordered single layer PdO(101) structure starts to develop under semirealistic reaction conditions.24,25 A recent study using surface X-ray diffraction (SXRD) with product detection also demonstrates that bulk-like PdO(101) layers develop on Pd(100) at near ambient pressure conditions and are also highly active for CO oxidation.8 Lastly, a recent in situ study using high-pressure X-ray photoelectron spectroscopy (HPXPS) reveals that thin oxide layers on Pd(111) are highly active for CO oxidation.26 While these prior studies provide evidence that Pd oxide layers can be quite active toward CO oxidation and can play an important role under realistic reaction conditions, the CO oxidation mechanism on well-defined Pd oxide surfaces is poorly understood. We have previously reported that CO oxidation is highly facile on a multilayer PdO(101) film during both TPRS27 and isothermal conditions28 in UHV. Using RAIRS, we also have recently presented evidence that Ocus atom vacancies promote CO oxidation on the PdO(101) surface under isothermal conditions.28 In the present study, we investigated CO oxidation on PdO(101) using density functional theory (DFT) calculations as well as TPRS and RAIRS measurements. We report detailed information about the mechanism for CO oxidation on PdO(101), and clarify the role that Ocus vacancies play in mediating this reaction.
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EXPERIMENTAL DETAILS
The experiments reported in this study were conducted in an ultrahigh vacuum (UHV) chamber with a typical base pressure of 2 × 10−10 Torr. The UHV chamber is equipped with a fourgrid retarding field analyzer for low energy electron diffraction (LEED) and Auger electron spectroscopy (AES), an ion source for Ar+ sputtering, a quadrupole mass spectrometer (QMS) used for TPD experiments, and a Fourier transform infrared spectroscopy (FTIR) system for RAIRS measurements. A single-stage differentially pumped chamber29 is also attached to the main UHV chamber which houses an inductively coupled RF plasma source that is used to generate atomic oxygen beams. The atomic oxygen beams are collimated by a quartz tube (6 mm in diameter) which separates the beam chamber and the main UHV chamber, and a mechanical shutter is used to control the flow of the beam into the UHV chamber. We use line-of-sight mass spectrometry to estimate the beam composition and flow rate and thereby ensure reproducibility in the atomic oxygen flux reaching the sample surface. 28648
dx.doi.org/10.1021/jp509383v | J. Phys. Chem. C 2014, 118, 28647−28661
The Journal of Physical Chemistry C
Article
assuming that the saturation coverage of CO2 adsorbed on PdO(101) at 85 K is 0.47 ML.35
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COMPUTATIONAL DETAILS The periodic plane wave DFT calculations reported in this paper were performed using the Vienna ab initio simulation package (VASP)36,37 with projector augmented wave (PAW)38 pseudopotentials provided in the VASP database. The Perdew− Burke−Ernzerhof (PBE) exchange-correlation functional39 was used with a plane wave cutoff of 400 eV. DFT-PBE predicts no band gap for bulk PdO, but experimentally PdO is known to be a small band gap semiconductor. Experimental band gap values vary from 0 to 2.67 eV in the literature depending on the measurement method.40 While PBE-DFT is well-known to underestimate band gaps, the Heyd−Scuseria−Ernzerhof (HSE)41 hybrid functional has been shown to accurately reproduce the band gap of oxides such as Cu2O, FeO, and NiO.42,43 Bruska and co-workers have reported a band gap of 0.8 eV using the HSE functional, which matches the experimental value from optical transmittance measurements.40,44 They also find excellent agreement between their HSE-DFT values and the experimental heat of formation and lattice parameters. The HSE functional can be easily modified by changing the amount of Hartree−Fock (HF) exact exchange. Bruska and co-workers have explored using this modification to increase the band gap to match an experimental conductivity derived band gap value of 1.5 eV,45 but the resulting heat of formation shows much larger deviation from the experimental values.40 Based on these results and the general positive results in the literature for HSE-derived band gaps for oxides, we apply the standard HSE06 functional formulation in all of the HSE calculations reported in this paper.46 We report in section S1 of the Supporting Information (SI) the details of our own bulk PdO calculations with PBE and HSE functionals. We have performed test calculations of CO adsorption on the PdO(101) surface with HSE, and the results are discussed in more detail below. Figure 1 illustrates the stoichiometric PdO(101) surface that is investigated in this study. Bulk crystalline PdO has a tetragonal unit cell and consists of square planar units of Pd atoms 4-fold coordinated with oxygen atoms. The bulkterminated PdO(101) surface is defined by a rectangular unit cell, where the a and b lattice vectors coincide with the [010] and [10̅ 1] directions of the PdO crystal, respectively. The stoichiometric PdO(101) surface consists of alternating rows of 3-fold or 4-fold coordinated Pd or O atoms that run parallel to the a direction shown in Figure 1. Thus, half of the surface O and Pd atoms are coordinatively unsaturated (cus). The areal density of each type of coordinatively distinct atom of the PdO(101) surface is equal to 35% of the atomic density of the Pd(111) surface. Hence, the coverage of cus-Pd atoms is equal to 0.35 ML, and each PdO(101) layer contains 0.7 ML of Pd atoms and 0.7 ML of O atoms. The PdO(101) surface was modeled by a rectangular 4 × 1 unit cell, with a corresponding 4 × 2 × 1 Monkhorst−Pack kpoint mesh. As in our prior studies47−49 the PdO(101) film was strained (a = 3.057 Å, b = 6.352 Å) to match the PdO(101) film structure resolved by Kan and Weaver.31,32 The PdO(101) slab was represented by four layers resulting in a 9 Å thick slab. The bottom layer is fixed, but other lattice atoms are allowed to relax until the forces are less than 0.03 eV/Å. As in our previous work, the underlying Pd(111) surface is not included since this would require a large unit cell due to the registry of the
Figure 1. Top and side views of the PdO(101) surface. The red and dark blue atoms represent O and Pd atoms, respectively. Rows of coordinatively unsaturated (cus) and 4-fold-coordinated (4f) Pd or O atoms are indicated. The vertical and horizontal arrows a and b represent the [010] and [1̅01] crystallographic directions of PdO.
PdO(101) film with the Pd(111) surface. We use a vacuum spacing of 20 Å, which is sufficient to eliminate spurious interactions in the surface normal direction. We determined the barriers and pathway for CO2 formation on the PdO(101) surface using the climbing nudged elastic band (cNEB) method.50 Vibrational frequencies were calculated with only the degrees of freedom associated with the CO molecule included. We performed calculations for selected configurations and find that including the motions of the neighboring Pd and O atoms in the normal-mode analysis has a negligible effect (
